Use spirv backend in libclc

[nikic]

Currently libclc relies on llvm-spirv from SPIRV-LLVM-Translator for the spirv-mesa3d targets. Having this kind of external dependency makes building this library challenging.

Now that we have an in-tree SPIRV backend, would it be possible to switch libclc to use it? I'm not familiar with what the requirements/differences here are. cc @frasercrmck @michalpaszkowski

[frasercrmck]

It's something I've been wondering about for a while but since I don't "use" the SPIR-V backend (and everyone who does is downstream AFAICT) I've not had any confidence that I wouldn't mess something up in doing so :)

Downstream in SYCL we do a lot with SPIR-V and are testing and improving SPIR-V backend so I'm hopeful.

[frasercrmck]

Let's say that adding the option to build with the SPIR-V backend (and check that it's not currently crashing) would be a good first step. But I wouldn't feel confident going ahead and changing the SPIR-V object from under the users' noses without some buy in.

[frasercrmck]

I hacked a quick thing into libclc, given #138751, and I can see that the SPIRV backend isn't ready to compile libclc:

fatal error: error in backend: unable to legalize instruction: %4639:iid(s1) = G_IS_FPCLASS %4638:fid(s32), 3 (in function: _Z11__clc_isnanf)

I don't know if it's just this instruction but I'm unable to easily work around it. There's already an issue for this: #72862. I'll ping there.